1. Field of the Invention
The present invention relates to a method of forming a crystal, and particularly to a method of forming a crystal by selectively growing monocrystal on a substrate having a surface with a low nucleation density.
The method of forming a crystal of the present invention is preferably used for electronic devices such as semiconductor integrated circuits, optical integrated circuits and the like, optical devices, magnetic devices, piezoelectric devices, surface acoustic devices and the like.
2. Related Background Art
Monocrystal films used for semiconductor devices and optical devices are generally formed by epitaxially growing monocrystal on a monocrystalline substrate.
For example, it is known that Si, Ge or GaAs is epitaxially grown on a Si monocrystalline substrate (silicon wafer) from a liquid phase, a vapor phase or a solid phase thereof. It is also known that a GaAs or GaAlAs monocrystal is epitaxially grown on a GaAs monocrystalline substrate.
The semiconductor films formed by the above methods are used for producing semiconductor devices, integrated circuits, light emitting devices such as semiconductor lasers, LED and the like.
On the other hand, very high speed transistors which use two-dimensional gases and superlattice elements which use quantum wells have recently actively been researched and developed. These devices can be realized by high precision epitaxial techniques such as MBE (molecular beam epitaxy) using super high vacuum, MOCVD (metal organic chemical vapor deposition) and the like.
Such epitaxial growth on a monocrystalline substrate requires conformity in lattice constant and thermal expansion coefficient between the monocrystalline material of the substrate and the epitaxially grown layer.
For example, although an Si monocrystalline film can be epitaxially grown on sapphire used as an insulator monocrystalline substrate, the crystal lattice defects caused by the difference between the lattice constants at the interface and the diffusion of the aluminum component of sapphire into the epitaxial layer are problems in application to electronic devices and circuits.
As described above, it is known that the conventional methods of forming a monocrystalline film by epitaxial growth are significantly dependent on the substrate material used. Mathews et al. investigated combinations of substrate materials and epitaxially grown layers (EPITAXIAL GROWTH, Academic Press, New York, 1975, edited by J. W. Mathews).
Presently, the size of a substrate is about 6 inches in the case of an Si wafer and the size of a GaAs or sapphire substrate is slightly increased.
In addition, since a monocrystalline substrate is produced at high cost, the cost per chip is increased.
The conventional methods of forming a monocrystalline layer which enables the formation of excellent devices have the problem that the kinds of materials for the substrate are limited to a very narrow range.
On the other hand, there have recently been active research and development of three-dimensional integrated circuits in which a semiconductor device is laminated on a substrate in the direction of a normal line thereof so that an increase in integration and diversification of the function can be achieved.
In addition, large area semiconductor devices such as solar cells, liquid crystal pixel switching transistors and the like in which elements are arrayed on inexpensive glass are increasingly actively researched and developed year after year.
Both types of devices commonly require the technique of forming a semiconductor film on an amorphous insulator and then forming electronic devices such as transistors or the like thereon. Particularly, there is a demand for the technique of forming high-quality monocrystalline semiconductors on an amorphous insulator.
In general, when a thin film is deposited on an amorphous insulator substrate composed of SiO.sub.2, the crystal structure of the deposited film is amorphous or polycrystalline due to the absence of long-distance order in the substrate material. The term "an amorphous film" represents a film assuming the state order is held within a short range such as a distance between adjacent atoms but is not held at a longer distance, and the term "a polycrystalline film" represents a film comprising monocrystalline grains having no particular crystal orientation and gathering with grain boundaries which separate the grains from each other.
For example, Si is deposited on SiO.sub.2 by a CVD process, amorphous silicon is formed at a deposition temperature of about 600.degree. C. or less, and polycrystalline silicon comprising grains having a size of several hundreds to several thousands .ANG. at a deposition temperature of 600.degree. C. or more. However, the grain size of the polycrystalline silicon formed is significantly dependent on the formation conditions.
Further, an amorphous or polycrystalline film is molten and solidified by using an energy beam generated from a laser or a rod heater to form a polycrystalline film comprising grains having a large size of several microns or millimeters (Single Crystal Silicon on Non-Single Crystal Insulators, Journal of Crystal Growth Vol. 63, No. 3, 1983 edited by G. W. Cullen).
As a result of measurement of electron mobilities of transistors formed on the thin films formed by the above method and having various crystal structures, the mobility of amorphous silicon was .about.0.1 cm.sup.2 /V.multidot.sec, the mobility of polycrystalline silicon comprising grains having a size of several hundreds .ANG. was 1 to 10 cm.sup.2 /V.multidot.sec, and the mobility of polycrystalline silicon formed by melting and solidification and comprising large grains was substantially the same as that of monocrystalline silicon.
These results show that the electrical properties of the device formed in a monocrystalline region within a crystal grain are significantly different from those of the device formed on a region including a grain boundary.
Namely, the film deposited on an amorphous substrate has an amorphous or polycrystalline structure, and the device formed on the deposited film has poor properties, as compared with the device formed on a monocrystalline layer. The application of such a device formed on a deposited film is limited to a simple switching element, a solar cell, a photoelectric conversion element or the like.
Methods of depositing a crystal layer on an amorphous substrate are roughly divided into the following two methods.
One method is a method in which an amorphous insulator (for example, SiO.sub.2) is coated on a monocrystal (for example, Si) used as a substrate and then partially removed to expose the surface of the ground monocrystal, and a monocrystalline region is then formed on the amorphous insulator layer by horizontal epitaxial growth from a vapor phase, solid phase or liquid phase using the exposed ground monocrystal surface as a seed crystal.
The other method is a method of growing a crystal thin film directly on a substrate without using a monocrystal as a substrate.
As described above, since the surface of an amorphous substrate has no long-distance order, as the surface of a monocrystalline substrate, but holds short-range order only, the deposited thin film at best has a polycrystal structure in which grain boundaries are randomly present. In addition, since both the long-distance order and anisotropy defining crystal orientation (orientation in the normal direction of the substrate and in the surface) are absent in the surface of an amorphous substrate, the crystal orientation of a layer formed on the substrate cannot be controlled.
Namely, a problem of deposition of a monocrystal on an amorphous substrate is to establish a technique of controlling boundary positions and crystal orientation.
A description will now be given of conventional methods of controlling boundary positions and crystal orientation and the problems thereof.
In regard to the control of boundary positions, EP244081A1 publication discloses that boundary positions can be determined by artificially previously defining nucleation positions.
In this technique, for example, Si.sub.3 N.sub.4 is localized on SiO.sub.2, Si monocrystal are grown from Si.sub.3 N.sub.4 serving as nucleation sites, and grain boundaries are formed by collision between the crystals grown from adjacent nucleation sites, thereby determining boundary positions.
Another technique is also proposed in which a non-monocrystalline material serving as primary seed is previously patterned instead of nuclei which are spontaneously produced at nucleation positions, the material is changed to monocrystal serving as seed crystals by employing aggregation (Japanese Patent laid-Open No. 1-132117).
The plane orientation (crystal orientation in the direction vertical to the substrate) of the crystals grown by the above techniques is determined by the factors such as stabilization of interfacial energy at the amorphous interface with the substrate, stabilization of free surface energy, relaxation of internal stress and so on during the generation of nuclei or aggregation of primary seeds. However, it is difficult to obtain complete orientation in a single direction, and main plane orientation and other orientations are frequently mixed. Particularly, one in-plane crystal orientation is not determined because the nucleation surfaces or aggregation surfaces are amorphous and do not have anisotropy.
On the other hand, H. I. Smith first showed that anisotropy caused by irregularity is artificially provided on the surface of an amorphous substrate by lithography so that the crystal orientation of KCl deposited on the substrate can be controlled, and named this technique "graphoepitaxy" (H. I. Smith and D. C. Flanders, Applied Physics Letters Vol. 32, pp. 349, 1978) (H. I. Smith, U.S. Pat. No. 4,333,792, 1982).
It was later confirmed that an artificial relief pattern formed on the surface of a substrate affects the crystal orientation in the growth of grains in a Ge thin film (T. Yonehara, H. I. Smith, C. V. Thompson and J. E. Palmer, Applied Physics Letters, Vol. 45, pp. 631, 1984) and in the initial growth of Sn (L. S. Darken and D. H. Lowndere, Applied Physics Letters Vol. 40, pp. 954, 1987).
Although it was found that graphoepitaxy has an effect on the orientation of each of separate KCl or Sn crystals deposited in an initial stage, there have also been reports of continuous layers, e.g., an Si layer formed by laser annealing crystal growth after deposition (M. W. Ceis, D. A. Flanders and H. I. Smith, Applied Physics Letters, Vol. 35, pp. 71, 1979) and a Ge layer formed by solid phase growth (T. Yonehara, H. I. Smith, C. V. Thompson and J. E. Palmer, Applied Physics Letters, Vol. 45, pp. 631, 1984).
However, in the cases of Si, Ge layers, although orientation is controlled to some extent, a crystal group is arranged in a mosaic pattern, and grain boundaries are randomly present between crystals having slightly different crystal orientations. It is thus impossible to obtain monocrystal uniformly arranged in a large area.
The reason for this is that the three-dimensional crystal orientations of respective crystals are not completely the same, and the nucleation positions cannot be controlled by a surface relief pattern.